The present invention relates to a semiconductor light emitting device. Typically, the invention relates to a light emitting diode having a light emitting layer made of AlGaInP materials.
It is noted that the term "AlGaInp materials" herein refers to those in which mixed crystal ratios x, y of (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P are changed in the ranges of 0.ltoreq.x, y.ltoreq.1.
The (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material exhibits lattice matching with a GaAs substrate at an In mixed crystal ratio of y=0.51. Moreover, at the In mixed crystal ratio of y=0.51, the material turns to the direct transition type with the Al mixed crystal ratio in the range of x=0-0.7, where light emission of high brightness can be obtained over a wide wavelength region from red to green. As a result, the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material has recently begun to be widely used as a material of light emitting diodes. As such an (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P family light emitting diode, for example as shown in FIG. 8, there has been known one in which an n-type GaAs buffer layer 211, an n-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 203, a non-doped (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P active layer 210, a p-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 205 and a p-type GaP current spreading layer 206 are stacked one after another on an n-type GaAs substrate 212. In order to effectively entrap injection carriers into the active layer 210, the band gap of the clad layers 203, 205 is set larger than that of the active layer 210 (DH (double hetero) structure). In addition, an n-side electrode 207 is provided on a bottom surface of the GaAs substrate 212, and a p-side electrode 208 is provided on a top surface of the current diffused layer 206. Since the In mixed crystal ratio y is set to y=0.51, which falls in the lattice matching with the GaAs substrate, the crystallinity of the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P materials (active layer 210 and clad layers 203, 205) that contributes to the light emission becomes better. As a result of this, as can be understood from an energy band view of FIG. 9A, the vicinity of the bottom (energy value Ec) of the conduction band of the active layer 210 and the vicinity of the top (energy value Ev) of the valence band are both parabolically shaped, and as shown in FIG. 9B, peaks P.sub.10, P.sub.20 of the state density G(E) of carriers in the conduction band and the valence band are approximate to band ends Ec, Ev of these bands, respectively. Therefore, the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P active layer 210 itself is enabled to exhibit a relatively high internal quantum efficiency (which refers to an efficiency at which electricity is converted into light at around p-n junctions).
However, with the structure of FIG. 8, since the band gap of the GaAs substrate 212 is 1.42 eV, emitted light of red to green is absorbed, so that light output would lower to less than a half, as a problem. In the case where light emitting material is made from GaP, GaAsP, AlGaAs or the like, since the GaAs substrate is transparent to the light emission wavelengths, there has been no possibility of occurrence of a problem due to light absorption by the substrate. However, in the case where the light emitting material is made from (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P, such light absorption by the substrate would occur as far as a GaAs substrate is used, so that external quantum efficiency thereof (which refers to an efficiency at which light is taken out externally; sometimes, referred to simply as "efficiency" or "light emission efficiency") would lower.
To avoid the light absorption by the substrate, there has been proposed a light emitting diode, as shown in FIG. 10, in which an n-type GaAs buffer layer 311, an n-type distributed Bragg reflection (DBR) layer 313, an n-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 303, a non-doped (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P active layer 310, a p-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 305 and a p-type GaP current spreading layer 306 are stacked one after another on an n-type GaAs substrate 301 (Appl. Phys. Lett., vol. 61, No. 15 (1992), pp. 1775-1777). In this light emitting diode, the DBR layer 313 formed by alternately combining two types of semiconductor layers of different refractive indexes in appropriate layer thickness is provided between the GaAs substrate 301 and the n-type clad layer 303 so that light emitted by the active layer 310 will be reflected upward by the DBR layer 313 so as not to reach the GaAs substrate 301 side. Further, there has been proposed a light emitting diode, as shown in FIG. 11, which has been fabricated through the steps of stacking an n-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 403, a non-doped (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P active layer 410, a p-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 405 and a p-type GaP current spreading layer 406 one after another on an unshown GaAs substrate, removing the GaAs substrate by etching and then joining a GaP substrate (with band gap 2.27 eV) 414, transparent to light emission wavelengths of red to green, with an exposed surface (junction part) 420 of the clad layer 403 (Appl. Phys. Lett., vol. 64, No. 21 (1994), pp. 2839-2841).
However, with the light emitting diode of FIG. 10, all of the light emitted downward from the active layer 310 could not be reflected by the DBR layer 313, so that part of the light will be transmitted by the DBR layer 313 and absorbed by the GaAs substrate 301. As a result, this light emitting diode could only be enhanced in light emission efficiency to around 1.5 times that of the light emitting diode of FIG. 8.
Meanwhile, the light emitting diode of FIG. 11 would encounter a difficulty in the technique of joining the GaP substrate 414, being unsuitable for mass production.
Under these backgrounds, there has been thought hitherto a means of growing the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material not on a GaAs substrate but on a substrate which is transparent to emission light wavelengths (650-550 nm) of the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material, typically on the aforementioned GaP substrate (with band gap 2.27 eV). That is, as shown in FIG. 6, there has been thought a light emitting diode in which an n-type GaInP buffer layer 104, an n-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 103, a non-doped (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P active layer 110, a p-type (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P clad layer 105 and a p-type GaP current spreading layer 106 are grown on an n-type GaP substrate 101.
However, as shown in FIG. 5, in the vicinity of a GaP lattice constant of 5.451 .ANG., there exists no region that permits direct transition of the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material. Therefore, even if the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P material showing lattice matching with the GaP substrate has been grown on the GaP substrate 101, light emission of high efficiency can not be expected. On the other hand, due to the fact that the lattice constant 5.653 .ANG. of the GaAs substrate is about 3.6% greater than the lattice constant 5.451 .ANG. of the GaP substrate, when the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P materials 103, 110 and 105 showing lattice matching with the GaAs substrate have been grown on the GaAs substrate 101, so-called misfit dislocations (dislocations due to lattice mismatching) would increase in the grown (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P materials 103, 110 and 105 so that non-radiative recombination centers would increase with the transition probability of light emission reduced, even though the GaInP buffer layer 104 is interveniently provided so as to relieve the lattice constant difference. That is, because crystals of different lattice constants are grown on the substrate, there would occur a disturbance in the periodicity of the grown crystal lattice so that a definite forbidden band can not exist. As a result of this, as shown in FIG. 7A, neither the vicinity of the bottom (Ec) of the conduction band of the active layer 110 nor the vicinity of the top (Ev) of the valence band is parabolically shaped, but each of them has a tail of about several tens meV so that, as shown in FIG. 7B, tip ends Ec#, Ev# of the tails (not necessarily definite in position) fall away from the peaks P.sub.10, P.sub.20 of the state density G(E) of carriers in the conduction band and the valence band, respectively. As a result of this, injected carriers are unlikely to recombine in the vicinity of the band ends Ec, Ev, so that the transition probability of light emission becomes smaller. Therefore, for the direct transition type light emitting diode that has been fabricated by growing on the GaP substrate 101 the (Al.sub.x Ga.sub.1-x).sub.1-y In.sub.y P materials 103, 110 and 105 showing lattice matching with the GaAs substrate, it would be difficult to obtain light emission of high efficiency. Actually, its light emission efficiency would lower by two orders or more, compared with the light emitting diode of FIG. 8.
Thus, even if the semiconductor substrate is transparent to the light emission wavelengths, a semiconductor light emitting device in which a light emitting layer (active layer) is grown in a state of lattice mismatching with the semiconductor substrate has a problem that light emission of high efficiency can not be obtained.